What about doing the opposite of your second idea? Instead of putting an "island" in it, put a "lake" in it (with the beta) and then have the terrarium part surrounding it. That way, you could just pick up the lake and clean it, without disturbing the surroundings. You would need something like a double lining, so that when you pull out the lake, there is still something there to hold the soil etc in place, but that wouldn't be too hard to find.